Over the last several years, plasma etching has increasingly replaced wet chemical etching in the fabrication of integrated circuits and semiconductor devices. Plasma etching is a dry etch process that is typically carried out in a metal reaction chamber. During the plasma etching process, a gas is pumped into the reaction chamber and is excited by a radio frequency generator to a plasma state. The plasma generated in the chamber may be used, for example, to etch a thin metal film on a semiconductor substrate.
During the plasma etch process, reaction by-products adhere to the interior wall or walls of the reaction chamber. After a certain period of time, the adhering reaction by-product film randomly peels off from the interior wall or walls of the reaction chamber, thereby generating film particles in the chamber. If the concentration or density of these particles stays below a certain threshold value, i.e., which is considered a “normal chamber condition”, the semiconductor wafers processed in the chamber are not negatively effected. If, however, the concentration or density of these particles exceeds the threshold value, i.e., which is considered an “abnormal chamber condition,” the semiconductor wafers processed in the chamber can be negatively effected, as the high particle concentrations in the chamber can cause defects in the semiconductor structures being formed on the wafers.
One existing method for monitoring the concentration of particles generated in the reaction chamber by peeling reaction by-product film, is to perform an off-line particle monitor. This method involves periodically placing a blank semiconductor wafer in the reaction chamber and operating the chamber according to a preset process or recipe similar to an actual wafer production recipe including the use of a flow gas, servo pressure, and RF power. The wafer is then removed from the chamber and the number of particles deposited on the wafer is counted. If the number of particles on the wafer exceed a certain value, an abnormal chamber condition exists and a maintenance cleaning must be performed on the chamber to remove the adhering film that is peeling off from the interior wall or walls of the reaction chamber to provide normal chamber processing conditions.
Another method presently used for monitoring the concentration of particles generated in the reaction chamber by the peeling film, is to perform a periodic defect inspection using a defect inspection tool to scan product wafers directly after they are removed from the chamber. This process takes a considerable amount of time to perform. Since the time during which the inspection is taking place is used to process additional semiconductor wafers, this process does not provide and “immediate” indication of an abnormal process chamber.
Thus, a major disadvantage of both of the above methods is that they are both performed while additional wafers are still being processed in what could be an abnormal process chamber. If an abnormal reaction chamber condition is detected, typically, many wafers have already been processed under these abnormal chamber conditions, which may cause defects in the semiconductor structures being formed on the wafers. Additionally, because of the randomness of the film peeling, these test methods may not always be needed. If the chambers are not utilized during the performance of the test method to avoid the possibility of making defective products, and the test indicates a normal chamber, the downtime decreases the utilization of the etching apparatus.
Accordingly, what is needed is a method for in situ monitoring particles generated from reaction by-product films peeling from the interior wall or walls of a reaction chamber of a plasma etching apparatus.